The present invention relates to methods by which L-tyrosine derived diphenol monomers may be synthesized with significantly improved yield and purity. The present invention also relates to new tyrosine-derived diphenol monomers and polymers prepared therefrom.
Diphenols are monomeric starting materials for polycarbonates, polyiminocarbonates, polyarylates, polyurethanes, and the like. Commonly owned U.S. Pat. No. 5,099,060 discloses amino acid-derived diphenol compounds, useful in the polymerization of polycarbonates and polyiminocarbonates. The resulting polymers are useful as degradable polymers in general, and as tissue compatible bioerodible materials for medical uses, in particular. The suitability of these polymers for this end use application is the result of their polymerization from diphenols derived from the naturally occurring amino acid, L-tyrosine. Commonly owned U.S. Pat. No. 5,216,115 discloses polyarylates polymerized from L-tyrosine derived diphenols that are also useful as tissue compatible bioerodible materials for medical uses.
The L-tyrosine derived diphenol monomers are disclosed by the foregoing patents as being prepared by carbodiimide mediated coupling reactions, as disclosed in Bodanszky, Practice of Peptide Synthesis (Springer-Verlag, New York, 1984) at page 145. The purification of the L-tyrosine derived diphenols has proved to be difficult. The diphenol crystallizes slowly, precipitating as a water-immiscible oil contaminated with side products of the carbodiimide coupling agent. It has since been discovered that the purity and yield of the diphenol could be improved significantly by the addition of an auxiliary nucleophile such as 1-hydroxybenzotriazole hydrate (HOBt).
Although the purity of the crude diphenol improves significantly, the final purification of the compound remains problematic. A major contaminant is the urea by-product of the carbodiimide. For example, the most commonly available, lowest cost carbodiimide, dicyclohexylcarbodiimide (DCC) has proved highly effective in mediating the formation of L-tyrosine derived diphenol compounds, but produces a by-product, dicyclohexylurea (DCU) that is distributed between the aqueous phase and the water-immiscible diphenol containing phase. The complete removal of DCU by extraction and/or precipitation techniques has not been achieved, which is consistent with previous reports indicating that DCU is difficult to remove from a variety of materials. Consequently, column chromatography is required for the complete purification of L-tyrosine derived diphenol compounds prepared by DCC-mediated coupling reactions.
Column chromatography purification techniques are satisfactory for the small scale preparation of L-tyrosine derived diphenol monomers. However, upon scale-up, several disadvantages become evident, including a reduction in yield, the need to use large amounts of organic solvents as eluents, and the inherent difficulties of large scale column chromatography. A method is needed by which L-tyrosine derived diphenol compounds may be prepared without difficult to remove by-products. A method by which such by-products may be readily removed would also be desirable.
WO 93/25,571 describes a method by which peptides containing from 2 to 10 amino acids are synthesized by carbodiimide-mediated amino acid coupling reactions. A solvent in which the urea by-product of the carbodiimide is insoluble is utilized as the reaction solvent, so that the urea by-product precipitates and is removed by filtration. However, the process is not applicable to the synthesis of the L-tyrosine-derived diphenols of this application since the urea by-product remains soluble in the reaction solvent to an appreciable extent. Accordingly, there remains a need for an L-tyrosine coupling method in which carbodiimide by-product formation is prevented or, in the alternative, the removal of such by-products is facilitated.